It shouldn't be present anywhere. The promote.dat file is created once the software determines that the standby should be promoted. What you may have been running into is a bug, an indefinite loop of promotion/demotion:
1) Portal 1 is primary and portal 2 is standby. Portal 1 has the promote.dat file. If this file is present in the db directory on standby, it will be promoted to primary. At some point the standby came back from being down and the file was present in the db directory on primary.
2) When standby comes back, it needs to get a db snapshot from primary. This is basically copying the db directory from portal 1 to portal 2.
3) When portal 2 gets the db directory, the promote.dat file tells it to promote itself to primary.
4) Logic in portal 1 is validating whether portal 2 is standby. When it isn't, it backs up the db directory to dbXXXX and sends over another db directory.
5) Now the deployment is back at step 3, and so on.
There are two issues: 1) the promote.dat file is left on primary and 2) the promote.dat file is moved over to standby during the db snapshot. I'm not sure how 1 could have happened but 2 is easily handled by deleting the file when we create the db snapshot. We plan to do this within the software.